Because the run could be segmented (done in multiple dungeons and not the same run), speed up by 1.01, the toolbox timer could be manipulated, but mainly because the rules state so.

"2. For a record to be valid the time must be faster than or equal to the current record displayed on the main page. Time is determined by the in-game /age command. The /age command should be visible and clearly readable in at least one video or screenshot. Some missions are an exception to this rule; check the Mission Record Rules for details."

"2. For multiple-level dungeons, the completion time is considered the difference between account or character /age at the beginning and end of the dungeon. Start /age must be entered outside the dungeon, before zoning into it, or inside, just after zoning in."